In many industrial processes, hot gases under pressure are produced. These gases, usually consisting primarily of air or air depleted in oxygen, are often vented to the atmosphere after passage through pollution control equipment, such as scrubbers, resulting in the waste of the energy contained therein. It is becoming common to recover energy from such gases by expanding the gas through a gas expansion engine in the form of an expansion turbine, denoted an expander. The expander is used to drive equipment such as air compressors and electric power generators. This invention relates to a method for synchronizing a generator driven by an expander to an electric power grid so that the output of the generator can be supplied to the grid. An electric power grid consists of numerous electricity producers and numerous electricity users, all connected by a common network of electrical power transmission lines.
This invention is especially adapted for use with an expander used to recover energy contained in flue gas from a regenerator used in the fluid catalytic cracking of hydrocarbons. Hydrocarbon feed and catalyst are introduced into a reactor in which the cracking reactions take place to produce hydrocarbon products. As a result of the reactions, the catalyst acquires a coating of carbonaceous matter, usually referred to as coke, which interferes with the effectiveness of the catalyst. The normal procedure in a fluid catalytic cracking plant is to continously withdraw catalyst, treat it to remove coke, and return it to the reactor. Treatment is accomplished by subjecting the catalyst to a high temperature environment in a pressure vessel called a regenerator. The high temperature environment is comprised of air serving as a fluidizing medium for the catalyst and as a source of oxygen for combustion of the accumulated surface deposits (coke). Catalyst is removed from the regenerator gas before the gas passes through the expander, usually by cyclone separating means within the regenerator.
When an expander is utilized to drive a generator, the speed of the generator is directly dependent on expander speed until the generator is connected to a grid. In order to connect a generator to a grid, the frequency of the electrical current produced by the generator must match the frequency of the electrical current flowing in the grid within about plus or minus 0.25 Hz. Because frequency is directly dependent on speed, the expander must be operated in a relatively narrow speed range, about 25 RPM wide. Since the pipeline supplying gas to an expander and the control valve controlling gas flow in that pipeline is normally quite large, on the order of four feet or more in diameter, it is difficult to attain a sufficiently precise expander speed during start-up. That the pressure upstream of the expander control valve may be variable adds to the difficulty. Due to hysteresis, inertia, and worn and sticky components, the smallest increment of movement of a large valve may produce a change in expander speed that is larger than 25 RPM. In the example of a fluid catalytic cracking prdcess, pressure in the regenerator during expander start-up is controlled by adjustment of a control valve almost as large as the expander control valve and subject to the same precision difficulties. Thus, as process conditions vary and the regenerator pressure control valve responds in an attempt to maintain regenerator pressure at a pre-set value, there are fluctuations at the expander valve inlet which usually result in a change in flow through the expander. The instant invention provides a method and apparatus to control expander speed precisely, so that a generator driven by it can be connected to a grid.
U.S. Pat. Nos. 3,401,124 (Goulden) and 3,247,129 (Roelofsen) may be consulted for more background on the use of expanders with fluid catalytic cracking processes. In both, a steam generator is used to recover energy from the gas stream additional to that recovered by use of an expander. Roelofsen shows use of an expander to drive an air compressor which provides air to the regenerator and a valve 51 which is analogous to pressure control valve 6 of the instant Drawing. ln normal operation, both valves open further to vent excess regenerator pressure and close further when pressure falls. An additional effect in the case of Roelofsen valve 51 is that its movement also affects regenerator pressure by changing air compressor speed. Goulden shows use of an expander to drive an air compressor and a motor-generator, but does not address the problem of frequency synchronization before connecting the generator to the plant power system. The arrangement of valves which includes valves 64, 64', and 65 in Goulden appears to be similar to valves 6 and 7 of the instant Drawing. There is a bypass line around the expander in Goulden to which flow is switched by a three-way valve; there is no pressure control loop associated with this line.
In U.S. Pat. Nos, 3,855,788 and 3,777,486, Damratowski shows an expander driving a generator and air compressor and utilizing the feature of two large control valves as mentioned above. There is also shown a steam turbine connected to the expander, generator, and air compressor, referred to as a starting steam turbine. Though not mentioned by Damratowski, such a steam turbine would normally be equipped with sufficiently precise speed controls so that it can be used in starting up the generator and adjusting its output frequency to match a grid. Damratowski also shows a relatively small bypass line containing a control valve, which is similar to that employed in the instant invention. However, that line starts downstream of the expander control valve rather than upstream as in the instant invention and is used for the purpose of preventing an overspeed trip rather than to sychronize speeds during start-up as in the instant invention. Further, the hand control valve of the instant invention is not shown. U.S. Pat. No. 4,188,792 (Schaible) may be consulted for additional background on regulation of turbines, in that case steam-driven turbines.